Lens Arrays for Pattern Projection and Imaging

ABSTRACT

A method for imaging includes focusing optical radiation so as to form respective first and second optical images of a scene on different, respective first and second regions of an array of detector elements. The focused optical radiation is filtered with different, respective first and second passbands for the first and second regions. A difference is taken between respective first and second input signals provided by the detector elements in the first and second regions so as to generate an output signal indicative of the difference.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication 61/419,891, filed Dec. 12, 2010. It is related to anotherU.S. patent application, filed on even date, entitled “Projection andImaging Using Lens Arrays.” Both of these related applications areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to optical projection andimaging, and specifically to devices and methods that use arrays oflenses to enhance the performance and characteristics of projection andimaging systems.

BACKGROUND

In most optical imaging and projection systems, the optical elements arearranged in series along a single optical axis. Some systems, however,use arrays of lenses arranged side by side. The best-known arrangementof this sort is the “fly's eye” lens array, which is generally used toachieve uniform irradiance in projection optics.

Lens arrays are also used in some imaging devices. For example, U.S.Pat. No. 7,700,904, whose disclosure is incorporated herein byreference, describes a compound-eye imaging device, which comprises nineoptical lenses arranged in a matrix array of three rows and threecolumns, and a solid-state imaging element for capturing unit imagesformed by the optical lenses. A stray light blocking member having arectangular-shaped window is provided on the capture zone side of theoptical lenses to block incident lights in a range outside eacheffective incident view angle range of each optical lens.

In general, the optics used in an imaging device are designed to form asingle image on an image sensor. In some applications, however, multipleimages may be superimposed. Such a scheme is described, for example, byMarcia et al., in “Superimposed Video Disambiguation for Increased Fieldof View,” Optics Express 16:21, pages 16352-16363 (2008), which isincorporated herein by reference. The authors propose a method forincreasing field of view (FOV) without increasing the pixel resolutionof the focal plane array (FPA) by superimposing multiple sub-imageswithin a static scene and disambiguating the observed data toreconstruct the original scene. According to the authors, thistechnique, in effect, allows each sub-image of the scene to share asingle FPA, thereby increasing the FOV without compromising resolution.

Various methods are known in the art for optical 3D mapping, i.e.,generating a 3D profile of the surface of an object by processing anoptical image of the object. This sort of 3D map or profile is alsoreferred to as a depth map or depth image, and 3D mapping is alsoreferred to as depth mapping.

Some methods of 3D mapping are based on projecting a laser specklepattern onto the object, and then analyzing an image of the pattern onthe object. For example, PCT International Publication WO 2007/043036,whose disclosure is incorporated herein by reference, describes a systemand method for object reconstruction in which a coherent light sourceand a generator of a random speckle pattern project onto the object acoherent random speckle pattern. An imaging unit detects the lightresponse of the illuminated region and generates image data. Shifts ofthe pattern in the image of the object relative to a reference image ofthe pattern are used in real-time reconstruction of a 3D map of theobject. Further methods for 3D mapping using speckle patterns aredescribed, for example, in PCT International Publication WO 2007/105205,whose disclosure is also incorporated herein by reference.

Other methods of optical 3D mapping project different sorts of patternsonto the object to be mapped. For example, PCT International PublicationWO 2008/120217, whose disclosure is incorporated herein by reference,describes an illumination assembly for 3D mapping that includes a singletransparency containing a fixed pattern of spots. A light sourcetransilluminates the transparency with optical radiation so as toproject the pattern onto an object. An image capture assembly capturesan image of the pattern on the object, and the image is processed so asto reconstruct a 3D map of the object.

SUMMARY

Embodiments of the present invention that are described hereinbelowprovide improved methods and apparatus for light projection and imagingusing lens arrays.

There is therefore provided, in accordance with an embodiment of thepresent invention, imaging apparatus, which includes an image sensor,including an array of detector elements, and objective optics, which areconfigured to focus optical radiation and are positioned so as to formrespective first and second optical images of a scene on different,respective first and second regions of the array. First and secondoptical filters, having different respective first and second passbands,are positioned so as to filter the optical radiation focused by thefirst and second lenses onto the first and second regions, respectively.A subtracter is coupled to take a difference between respective firstand second input signals provided by the detector elements in the firstand second regions and to generate an output signal indicative of thedifference.

Typically, the objective optics are arranged so that the first andsecond optical images contain a common field of view. In a disclosedembodiment, the objective optics include first and second lenses, whichare configured to form the first and second optical images,respectively.

In one embodiment, the subtracter is configured to take the differenceby subtracting digital pixel values from the first and second regions.Alternatively or additionally, the image sensor includes an integratedcircuit chip, and the subtracter includes an analog component on thechip.

In some embodiments, the apparatus includes a projection module, whichis configured to project a pattern onto the scene at a wavelength in thefirst passband, while the optical radiation focused by the objectiveoptics includes ambient background radiation in both the first andsecond passbands, whereby the second input signal provides an indicationof a level of the ambient background radiation for subtraction from thefirst input signal. The apparatus may include a processor, which isconfigured to process the output signal so as to generate a depth map ofthe scene responsively to the pattern appearing in the first opticalimage.

There is also provided, in accordance with an embodiment of the presentinvention, imaging apparatus, which includes an image sensor, includingan array of detector elements, and a plurality of lenses, which areconfigured to form respective optical images of respective portions of ascene on different, respective regions of the array along respectiveoptical axes. Diverting elements are fixed to respective surfaces of atleast two of the lenses and are configured to deflect the respectiveoptical axes of the at least two of the lenses angularly outwardrelative to a center of the image sensor.

In a disclosed embodiment, the diverting elements include diffractivepatterns that are fabricated on the respective surfaces of the at leasttwo of the lenses, wherein the diffractive patterns may define Fresnelprisms.

In some embodiments, the lenses have respective individual fields ofview, and the apparatus includes a processor, which is configured toprocess an output of the image sensor so as to generate an electronicimage having a combined field of view encompassing the different,respective portions of the scene whose optical images are formed by thelenses. The apparatus may include a projection module, which isconfigured to project a pattern onto the scene, wherein the processor isconfigured to process the electronic image so as to generate a depth mapof the scene responsively to the pattern appearing in the optical imagesof the respective portions of the scene.

There is moreover provided, in accordance with an embodiment of thepresent invention, a method for imaging, which includes focusing opticalradiation so as to form respective first and second optical images of ascene on different, respective first and second regions of an array ofdetector elements. The focused optical radiation is filtered withdifferent, respective first and second passbands for the first andsecond regions. A difference is taken between respective first andsecond input signals provided by the detector elements in the first andsecond regions so as to generate an output signal indicative of thedifference.

There is furthermore provided, in accordance with an embodiment of thepresent invention, a method for imaging, which includes positioning aplurality of lenses to form respective optical images of respectiveportions of a scene on different, respective regions of an array ofdetector elements along respective optical axes of the lenses. Divertingelements are fixed to respective surfaces of at least two of the lensesso as to deflect the respective optical axes of the at least two of thelenses angularly outward relative to a center of the array.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, pictorial illustration of a system forthree-dimensional (3D) mapping, in accordance with an embodiment of thepresent invention;

FIG. 2A is a schematic side view of an imaging module, in accordancewith an embodiment of the present invention;

FIG. 2B is a schematic frontal view of the imaging module of FIG. 2A;

FIG. 3A is a schematic side view of an imaging module, in accordancewith another embodiment of the present invention;

FIG. 3B is a schematic frontal view of the imaging module of FIG. 3A;

FIG. 4 is a schematic side view of an imaging module, in accordance withyet another embodiment of the present invention;

FIG. 5 is a schematic side view of a projection module, in accordancewith an embodiment of the present invention; and

FIG. 6 is a schematic side view of a projection module, in accordancewith another embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Embodiments of the present invention that are described hereinbelow uselens arrays in novel ways to enhance the performance of optical imagingsystems and of pattern projectors. In the disclosed embodiments, thelenses in an array are typically used together to form respective imageson the same image sensor, or to project different parts of a pattern.

The embodiments of the present invention that are described hereinbeloware useful particularly in pattern-based depth mapping. Therefore, forclarity and convenience of presentation, these embodiments are shown anddescribed in the context of the components of a depth mapping system.The principles of these embodiments, however, may also be used in otherelectronic imaging and optical projection applications, all of which areconsidered to be within the scope of the present invention.

FIG. 1 is a schematic, pictorial illustration of a system 20 for 3Dmapping, in accordance with an embodiment of the present invention. Inthis example, an imaging device 22 is configured to capture images andgenerate 3D maps of a scene. The scene here includes a user 28 of thesystem (who is thus, in this case, the “object” of the imaging device,as well as its operator). The depth information in the 3D maps may beused by a host computer 24 as part of a 3D user interface, which enablesthe user to interact with games and other applications running on thecomputer and with elements shown on a display screen 26. (This sort offunctionality is described, for example, in U.S. Patent ApplicationPublication 2009/0183125, whose disclosure is incorporated herein byreference.) This particular application of system 20 is shown here onlyby way of example, however, and the mapping capabilities of system 20may be used for other purposes, as well, and applied to substantiallyany suitable type of scenes and 3D objects.

In the example shown in FIG. 1, imaging device 22 comprises a projectionmodule 23, which projects a pattern of optical radiation onto the scene,and an imaging module 25, which captures an image of the pattern thatconsequently appears on the body of user 28 and other objects in thescene (not shown in the figure). The optical radiation that is used forthis purpose is typically, although not necessarily, in the infrared(IR) range, although visible or ultraviolet (UV) light may similarly beused. The terms “optical radiation,” “illumination,” and “light” areused synonymously in the present patent application and should beunderstood to include any or all of the IR, visible, and UV ranges.Module 23 may be designed to emit radiation in a narrow optical band,and a corresponding bandpass filter may be used in imaging module 25 inorder to reduce the amount of ambient light detected by the imagingmodule.

A processor, such as computer 24 or an embedded processor (not shown) indevice 22, processes the image of the pattern in order to generate adepth map of the body, i.e., an array of 3D coordinates, comprising adepth (Z) coordinate value of the body surface at each point (X,Y)within a predefined field of view. (In the context of an array ofimage-related data, these (X,Y) points are also referred to as pixels.)In the present embodiment, the processor computes the 3D coordinates ofpoints on the surface of the user's body by triangulation, based ontransverse shifts of the spots in the pattern, as described in theabove-mentioned PCT publications WO 2007/043036, WO 2007/105205 and WO2008/120217. This technique is referred to herein as “pattern-baseddepth mapping.” The functionality of a processor similar to that insystem 20 is further described, for example, in U.S. Patent ApplicationPublication 2010/0007717, whose disclosure is incorporated herein byreference.

For many practical applications, it is advantageous that imaging module25 have a wide field of view (FOV)—on the order of 90-120° or more inthe horizontal direction and 60-90° or more in the vertical direction.The imaging module is also expected to provide a clear image of thepattern over a wide range of ambient light conditions, including sceneswith a bright ambient background, which tends to reduce the contrast ofthe pattern in the captured images. On the other hand, power and safetyconsiderations limit the output intensity of projection module 23. Theembodiments that are described hereinbelow address these issues.

Imaging Module wit On-Board Ambient Cancellation

FIGS. 2A and 2B schematically illustrate an ambient light cancellationarrangement in imaging module 25, in accordance with an embodiment ofthe present invention. FIG. 2A is a side view showing an image sensor 30and other elements of module 25, while FIG. 2B is a frontal view.

Image sensor 30 may be, for example, a CMOS device or CCD, comprising anarray of detector elements 32. (For convenience of illustration, only asmall number of detector elements is shown in the figure, while inactuality the array typically contains a much larger number of elements,generally well in excess of one million.) The detector elements aretypically uniform in size and functionality over the matrix, but in thisembodiment they are divided into two regions 34 and 36. The regions maybe of the same size and shape, but for enhanced resolution of thespecific image captured by region 34, it may be advantageous that region34 is wider and thus includes a larger number of columns of detectorelements 32, for example, twice as many columns as region 36. Bothregions, however, have the same number of rows.

Objective optics, comprising lenses 42 and 44 form images of the sceneof interest on regions 34 and 36, respectively, of sensor 30. Typically,the lenses are designed and oriented so that regions 34 and 36 captureimages containing a common field of view. The image formed by lens 44may therefore be distorted in the horizontal direction in order to fitinto the narrower shape of region 36. Although, for the sake ofsimplicity, only a single lens is shown for each region, in practicearrays of multiple lenses may be used. Alternatively, a single lens (orgroup of lenses) with a suitable beamsplitting arrangement following thelens may be used to form the images on both of regions 34 and 36.Although lenses 42 and 44 are pictured as simple lenses, in practicecompound lenses may be used in this and all the other embodiments ofimaging module 25.

Lens 42 forms its image through a bandpass filter 38, which passes lightof the wavelength (typically IR) that is emitted by projection module23. Thus, region 34 senses an image of the pattern that has beenprojected by module 23 onto the scene of interest, along with whateverambient light is reflected from the scene in the passband of the filter.On the other hand, lens 44 forms its image through a bandpass filter 40,whose passband does not include the wavelength of projection module 23.Thus, region 36 senses only ambient background radiation from the scene.The passband of filter 40 may be selected to be near that of filter 38and of similar bandwidth, so that the image received by region 36 willprovide a faithful measure of the ambient light component in the imagereceived by region 34.

The ambient input signal from the rows of detector elements 32 in region36 is thus indicative of the level of the ambient component in the inputimage signal from the corresponding rows in region 34. A subtractertakes a difference between this ambient component from region 36 and theinput image signal generated by region 34 in order to generate an outputsignal representing to an electronic image of the pattern on the scenewith improved signal/background ratio and hence improved contrast.Because the pixels in regions 34 and 36 are row-aligned, the imagesignals from the two regions are inherently synchronized. When a rollingshutter is used in image sensor 30 (as is common in CMOS-type sensors),the simultaneous capture and readout of the pixels in the two regionsenables imaging module 25 to operate on non-static scenes without motionartifact.

One way to subtract the ambient component is to digitize the respectiveraw images from regions 34 and 36 and then subtract the digital pixelvalues using a suitable digital processor, such as computer 24 orhardware logic (not shown) in device 22. If region 36 is narrower thanregion 34, as shown in the figures, the pixel values in region 36 may beinterpolated before subtraction.

Since the points of view of lenses 42 and 44 are slightly different, theimages formed on regions 34 and also have slightly differentperspectives (although typically, the disparity is less than ¾ of thesensor width). It is beneficial to register the pixels of the image inregion 36 with those in region 34 prior to subtracting. Suchregistration can be achieved, for example, using optical flow techniquesthat are known in art. Prior to performing the subtraction, the image inregion 36 is interpolated onto the image in region 34 so as to representthe same pixels, same point of view and same overall optical gain. (Gaincorrection can be important, since filters 38 and 40 are different.)

Alternatively, as illustrated in FIG. 2B, the subtraction may be carriedout on the image sensor chip in the analog domain. For this purpose,regions 34 and 36 may have separate readout circuits, which are clockedso that each pixel in region 34 is read out at the same time as thecorresponding pixel in region 36. (The clock rates may be adjusted forthe difference in widths of the regions.) An analog component, such as adifferential amplifier 46 on the integrated circuit chip of the imagesensor serves as the subtracter in this case, subtracting the signallevel in region 36 from the signal level of the corresponding pixel inregion 34, so that the output from image sensor 30 is already correctedfor ambient background.

To improve accuracy of the results, image sensor 30 may also comprisecircuitry for performing local operations of optical flow and gainmodification, to ensure that the image signals from regions 34 and 36have locally the same point of view and gain.

Imaging Module with Wide Field of View

FIGS. 3A and 3B schematically illustrate an optical arrangement ofimaging module 25 that provides a wide field of view (FOV) in a compact,low-cost optical design, in accordance with an embodiment of the presentinvention. FIG. 3A is a side view showing an image sensor 50 and otherelements of module 25, while FIG. 3B is a frontal view. The principlesof this embodiment may be combined with those of the embodiments ofFIGS. 2A and 2B to give both wide FOV and ambient light rejection.

Image sensor 50 in this embodiment is divided into three regions 52, 54and 56, each with its own lens 62, 64, 66. Image sensor 50 may be astandard CMOS device or CCD. The lenses in this case are assumed to berefractive, although diffractive or combinations of refractive anddiffractive elements may alternatively be used for the same purpose.Furthermore, although the pictured embodiment divides the image sensorinto only three regions with respective lenses, a smaller or largernumber of regions and lenses may be used. In the embodiment shown inFIGS. 3A and 3B, lenses 62, 64 and 66 are arranged in a single row, thusexpanding the FOV of module 25 in one direction only (the horizontaldirection relative to the pages of these figures), but a two-dimensionalarray of lenses may likewise be used to expand the FOV in bothhorizontal and vertical directions.

Each of lenses 62, 64 and 66 has a respective FOV 72, 74, 76, as shownin FIG. 3A. At least two of the lenses, such as lenses 62 and 66, alsohave a diverting element, such as Fresnel prisms 68 and 70, fixed to oneof their surfaces, such as the front surface in the pictured embodiment.These diverting elements deflect the respective optical axes of thelenses on the front side of module 25 (i.e., the side facing toward thescene and away from image sensor 50) angularly outward relative to thecenter of image sensor 50. The angle of deflection of prisms 68 and 70is chosen so that fields of view 72 and 76 look outward and overlap onlyslightly at their inner borders with FOV 74.

As a result, module 25 has an overall FOV that is three times the widthof the individual FOV of each of the lenses. Each of regions 52, 54 and56 thus receives an image of a different part of the overall FOV,although it is possible that the images may overlap or that there may begaps between the images. An image processor, such as computer 24 or aprocessor embedded in device 22, may process the electronic image outputfrom sensor 50, if necessary, for proper blending and avoidance ofartifacts at the borders between the regions. In order to prevent straylight from passing between the lenses, separator walls 78 may beinterposed between the channels. Similar sorts of separators may be usedin the other embodiments described herein (but they are omitted from thefigures for the sake of simplicity).

Module 25 as shown in FIGS. 3A and 3B may achieve an overall FOV of90-120° with good image quality throughout (at least sufficient for thepurposes of system 20). Normally, good image quality over a FOV thiswide requires a large, costly lens, extending a large distance forwardfrom the image sensor. By using an array of lenses, on the other hand,the present embodiment achieves the same FOV with a much more compact,less costly design, and improved performance, since the FOV of each oflenses 62, 64, 66 is only one-third of the overall FOV. The use ofdiffractive technology for this purpose enables Fresnel prisms 68 and 70to be fabricated as part of the lenses themselves and avoids the needfor bulky refractive prisms or reflective elements.

FIG. 4 is a schematic side view of an optical arrangement of imagingmodule 25 that provides a wide field of view (FOV) in a compact,low-cost optical design, in accordance with another embodiment of thepresent invention. In this embodiment, too, an array of lenses 82 imagesthe scene of interest onto an image sensor 80, wherein each lenscaptures the image in a respective FOV 86, 88, 90, . . . . In this case,there are nine lenses 82 in a 3×3 array (although only three of thelenses are seen in the side view of FIG. 4), but again, larger orsmaller numbers of lenses may be used in either a one- ortwo-dimensional array. Alternatively, a single imaging lens may be used,with a suitable arrangement of beam combiners to multiplex andsuperimpose all of FOVs 86, 88, 90, . . . , through this same lens.

In contrast to the preceding embodiment, in the present embodiment allof lenses 82 cast their respective images of different areas of thescene onto a common area (typically the entire area) of the array ofdetector elements 32 in sensor 80. Thus, each of FOVs 86, 88, 90, . . .is imaged with the full resolution of sensor 80. The signal output bythe sensor, however, becomes a superposition of the images of all theindividual fields of view. An image processor, such as computer 24 or aprocessor embedded in device 22, separates out the individual images bya process of matched filtering of the output signal from sensor 80, inorder to reconstruct the specific images of the individual fields ofview. These specific images may be stitched together or otherwiseprocessed over the entire, combined FOV in order to provide an imagewith both wide FOV and high resolution.

The matched filtering performed by the image processor is based onoptical encoding of the images formed by lenses 82 with different,respective coding patterns. Various means may be used to perform thisencoding. For example, the individual image formed by each lens 82 maybe optically encoded, using means such as a respective coded aperture 84associated with the optical aperture of each lens 82. A coded aperture,as is known in the art, applies a predetermined spatial modulation tothe incoming light, which may be either an amplitude modulation or aphase modulation or a combination of the two. The resulting individualimage formed at the focus of the lens on image sensor 80 is then aconvolution of the result of geometrical optical imaging with theFourier transform of the aperture modulation function (representing thediffraction effects). Appropriate defocusing will thus cause ageometrical image of the aperture to appear as the image of a pointsource, and the modulated image will be a convolution of the aperturewith the original unmodulated image.

A set of mutually-orthogonal modulation functions may be chosen, with adifferent one of the functions applied by each of the differentapertures 84. The modulation functions are “mutually orthogonal” in thesense that the spatial correlation between any pair of the functions isinsignificant by comparison to the autocorrelation of each function withitself. Each function will then have a different, respectivedeconvolution kernel, which serves as a matched filter for the imageformed through the corresponding aperture 84. To extract the individualimage formed by each of lenses 82, the image processor performs asuccession of deconvolution operations using the respective kernels oralternatively solves simultaneously for all the individual images. Thedeconvolution of the individual images and reconstruction of thecombined FOV can be performed frame by frame, without reliance onprevious image frames or other temporal information.

As another alternative, projection module 23 may serve as the means forencoding the images by projecting a pattern chosen so that therespective partial patterns projected onto the scene in the differentfields of view 86, 88, 90, . . . are mutually orthogonal. In this case,these partial patterns themselves can serve as the matched filters. Theimage processor may perform a correlation computation between the imageoutput from sensor 80 and each of these partial patterns in order toextract the individual images of the partial patterns and find localpattern shifts as a function of position in each of the fields of view.The processor uses these pattern shifts in computing a depth map (withwide FOV), as described above.

As in the preceding embodiment, the use of the array of lenses 82, eachwith a moderate individual FOV, enables the system to achieve a wideoverall FOV at low cost, while maintaining a compact opticalconfiguration.

In an alternative embodiment, the optical arrangement shown in FIG. 4can be used to provide a sort of “zoom” functionality in patterndetection and depth mapping. In this embodiment, projection module 23initially projects a given pattern over the entire combined FOV of thearray of lenses 82. The image processor processes all of the individualimages, as described above, to give a wide-angle, low-resolution depthmap. The image processor may identify an object of interest in a certainsector of this depth map, within one or more of fields of view 86, 88,90, . . . . The image processor may then instruct projection module 23to adjust its optical configuration so that the pattern is projected,possibly with higher resolution, only into the limited sector in whichthe object is located.

The dimensions of the projected pattern in this “zoom” mode are lessthan or equal to the dimensions of the FOV of a single lens 82, whilethe pattern itself may be contained within the FOV of a single lens ormay overlap the fields of view of two or more of the lenses. As aresult, imaging module 25 will receive a single image of the pattern,via one or more of lenses 82, without other superimposed images of thepattern as in the wide-angle mode. The image processor may process thisindividual image in order to create an enhanced-resolution depth map ofthe object of interest. Thus, system 20 has simultaneously large FOV andhigh resolution and is able to choose a high-resolution sub-image fromwithin the large FOV.

Compact Pattern Projectors

When projection module 23 is required to project a pattern over a wideFOV, the projection lens may suffer from similar problems of size andcost as are encountered by the imaging lenses in the wide FOV imagingconfigurations described above. Furthermore, when coherent illuminationis used, large, wide-angle projection lenses can exacerbate eye safetyconcerns. The embodiments described below address these issues.

FIG. 5 is a schematic side view of projection module 23, in accordancewith an embodiment of the present invention. The module comprises alight source 91, such as a laser diode or LED. A condenser, such as alens 93, collimates or gathers, shapes, and directs the beam of lightemitted by the light source toward a transparency 95, which isinterposed in the beam and typically creates a pattern of light and darkspots. Light source 91, lens and transparency 95 together serve as apatterned illumination source.

Transparency 95 may comprise any of a wide range of optical components.The transparency may comprise, for example, a gray-level or otherwisepatterned optical element or a patterned microlens array (MLA), asdescribed in the above-mentioned PCT International Publication WO2008/120217, or any other suitable sort of patterned refractive ofdiffractive optical element (DOE).

The pattern created by this illumination source is projected onto thescene of interest by an array 100 of projection lenses 92, 94, 96, 98.These lenses each project a part of the overall pattern onto arespective FOV 102, 104, 106, 108, although there may be small overlapsor gaps between the respective parts of the pattern projected by theindividual lenses. Thus, the lenses of array 100 together project thepattern onto a wide overall FOV, typically 120° wide or more, with eachlens projecting its part of the pattern onto a different, respectivearea of the scene. The use of an array of small lenses of this sortmakes module 23 smaller and, typically, less costly to manufacture,while improving the performance of the individual lenses and thus of thewhole array. Although only one dimension of array 100 is shown in thisfigure, the projection lenses may be arrayed in two dimensions, i.e.,into the page as well as vertically in the view presented here.Furthermore, although the lenses in FIGS. 5 and 6 are shown in thefigures as simple lenses, in practice compound lenses may be used in allembodiments of projection module 23.

FIG. 6 is a schematic side view of projection module 23 in accordancewith another embodiment of the present invention. This embodiment sharesthe benefits of compactness, low cost, and improved performance with thepreceding embodiment, while adding the benefit of enhanced eye safety.In this case, projection lenses 112, 114, 116, 118, . . . in an array110 are all configured to project respective parts of the patterngenerated by an illumination source including a transparency 97 onto thesame FOV 120. (Transparency 97 may comprise any of the types oftransparencies mentioned above in reference to transparency 95.) Each ofthe projection lenses, in other words, projects its own pattern,generated by the corresponding part of transparency 97 (or equivalently,each of the lenses may be associated with its own pattern generatingelement) onto an area of the scene that is common to all of the lenses.The resulting pattern in FOV 120 is a superposition of all theindividual patterns cast by the lenses. Intricate patterns can becreated in this manner.

The eye safety is enhanced in this embodiment due to the followingconsideration: The light power that a projector can safely emit isdefined by the AEL (Accessible Emission Limit). For an extended source,the AEL is proportional to the angular subtense of the source, referredto as α, as well as by the f# of the projections lens, the FOV, and thearea of the source (in this case the area of transparency 97).Maintaining the same area of transparency 97, the same f# for theprojection lenses, and the same FOV, but dividing the projection lensinto an array of n×n lenses, for example, will provide a factor of nincrease in the AEL for the whole system. The reason for this increaseis that the aperture of each lens has an angular subtense that is 1/n ofthe original angular subtense, but there are n×n such apertures, so thatoverall the system can project n times more power while maintaining thesame level of eye-safety.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

1. Imaging apparatus, comprising: an image sensor, comprising an arrayof detector elements; objective optics, which are configured to focusoptical radiation and are positioned so as to form respective first andsecond optical images of a scene on different, respective first andsecond regions of the array; first and second optical filters, havingdifferent respective first and second passbands, which are positioned soas to filter the optical radiation focused by the first and secondlenses onto the first and second regions, respectively; and asubtracter, which is coupled to take a difference between respectivefirst and second input signals provided by the detector elements in thefirst and second regions and to generate an output signal indicative ofthe difference.
 2. The apparatus according to claim 1, wherein theobjective optics are arranged so that the first and second opticalimages contain a common field of view.
 3. The apparatus according toclaim 1, wherein the objective optics comprise first and second lenses,which are configured to form the first and second optical images,respectively.
 4. The apparatus according to claim 1, wherein thesubtracter is configured to take the difference by subtracting digitalpixel values from the first and second regions.
 5. The apparatusaccording to claim 1, wherein the image sensor comprises an integratedcircuit chip, and wherein the subtracter comprises an analog componenton the chip.
 6. The apparatus according to claim 1, and comprising aprojection module, which is configured to project a pattern onto thescene at a wavelength in the first passband, while the optical radiationfocused by the objective optics comprises ambient background radiationin both the first and second passbands, whereby the second input signalprovides an indication of a level of the ambient background radiationfor subtraction from the first input signal.
 7. The apparatus accordingto claim 6, and comprising a processor, which is configured to processthe output signal so as to generate a depth map of the sceneresponsively to the pattern appearing in the first optical image. 8.Imaging apparatus, comprising: an image sensor, comprising an array ofdetector elements; a plurality of lenses, which are configured to formrespective optical images of respective portions of a scene ondifferent, respective regions of the array along respective opticalaxes; and diverting elements, which are fixed to respective surfaces ofat least two of the lenses and are configured to deflect the respectiveoptical axes of the at least two of the lenses angularly outwardrelative to a center of the image sensor.
 9. The apparatus according toclaim 8, wherein the diverting elements comprise diffractive patternsthat are fabricated on the respective surfaces of the at least two ofthe lenses.
 10. The apparatus according to claim 9, wherein thediffractive patterns define Fresnel prisms.
 11. The apparatus accordingto claim 8, wherein the lenses have respective individual fields ofview, and wherein the apparatus comprises a processor, which isconfigured to process an output of the image sensor so as to generate anelectronic image having a combined field of view encompassing thedifferent, respective portions of the scene whose optical images areformed by the lenses.
 12. The apparatus according to claim 11, andcomprising a projection module, which is configured to project a patternonto the scene, wherein the processor is configured to process theelectronic image so as to generate a depth map of the scene responsivelyto the pattern appearing in the optical images of the respectiveportions of the scene.
 13. A method for imaging, comprising: focusingoptical radiation so as to form respective first and second opticalimages of a scene on different, respective first and second regions ofan array of detector elements; filtering the focused optical radiationwith different, respective first and second passbands for the first andsecond regions; and taking a difference between respective first andsecond input signals provided by the detector elements in the first andsecond regions so as to generate an output signal indicative of thedifference.
 14. The method according to claim 13, wherein focusing theoptical radiation comprises forming the first and second optical imagesso as to contain a common field of view.
 15. The method according toclaim 13, wherein focusing the optical radiation comprises positioningfirst and second lenses to form the first and second optical images,respectively.
 16. The method according to claim 13, wherein taking thedifference comprises subtracting digital pixel values from the first andsecond regions.
 17. The method according to claim 13, wherein the arrayof the detector elements is formed on an integrated circuit chip, andwherein taking the difference comprises subtracting the input signalsusing an analog component on the chip.
 18. The method according to claim13, and comprising projecting a pattern onto the scene at a wavelengthin the first passband, while the optical radiation focused by theobjective optics comprises ambient background radiation in both thefirst and second passbands, whereby the second input signal provides anindication of a level of the ambient background radiation forsubtraction from the first input signal.
 19. The method according toclaim 18, and comprising processing the output signal so as to generatea depth map of the scene responsively to the pattern appearing in thefirst optical image.
 20. A method for imaging, comprising: positioning aplurality of lenses to form respective optical images of respectiveportions of a scene on different, respective regions of an array ofdetector elements along respective optical axes of the lenses; andfixing diverting elements to respective surfaces of at least two of thelenses so as to deflect the respective optical axes of the at least twoof the lenses angularly outward relative to a center of the array. 21.The method according to claim 20, wherein fixing the diverting elementscomprise fabricating diffractive patterns on the respective surfaces ofthe at least two of the lenses.
 22. The method according to claim 21,wherein the diffractive patterns define Fresnel prisms.
 23. The methodaccording to claim 20, wherein the lenses have respective individualfields of view, and wherein the method comprises processing an output ofthe image sensor so as to generate an electronic image having a combinedfield of view encompassing the different, respective portions of thescene whose optical images are formed by the lenses.
 24. The methodaccording to claim 23, and comprising projecting a pattern onto thescene, and processing the electronic image so as to generate a depth mapof the scene responsively to the pattern appearing in the optical imagesof the respective portions of the scene.